Hydrogen is a preferred intermediate in the energy-conserving electron transport chain ofMethanosarcina barkeri

Abstract

Methanogens use an unusual energy-conserving electron transport chain that involves reduction of a limited number of electron acceptors to methane gas. Previous biochemical studies suggested that the proton-pumping F420H2dehydrogenase (Fpo) plays a crucial role in this process during growth on methanol. However,Methanosarcina barkeriΔfpomutants constructed in this study display no measurable phenotype on this substrate, indicating that Fpo plays a minor role, if any. In contrast, Δfrhmutants lacking the cytoplasmic F420-reducing hydrogenase (Frh) are severely affected in their ability to grow and make methane from methanol, and double Δfpo/Δfrhmutants are completely unable to use this substrate. These data suggest that the preferred electron transport chain involves production of hydrogen gas in the cytoplasm, which then diffuses out of the cell, where it is reoxidized with transfer of electrons into the energy-conserving electron transport chain. This hydrogen-cycling metabolism leads directly to production of a proton motive force that can be used by the cell for ATP synthesis. Nevertheless,M. barkeridoes have the flexibility to use the Fpo-dependent electron transport chain when needed, as shown by the poor growth of the Δfrhmutant. Our data suggest that the rapid enzymatic turnover of hydrogenases may allow a competitive advantage via faster growth rates in this freshwater organism. The mutant analysis also confirms the proposed role of Frh in growth on hydrogen/carbon dioxide and suggests that either Frh or Fpo is needed for aceticlastic growth ofM. barkeri.